The present invention relates to a thread control device for a looper thread of a chain stitch sewing machine.
A thread control device is known from German Auslegeschrift No. 24 15 991 corresponding to U.S. Pat. No. 3,903,821. Generally, the problem with such thread control devices is that, if the thread breaks in the region of the rotation take-up disc, the looper thread is caught up and wound up by said thread take-up disc. By the time the operator has noticed the break, a large quantity of thread has usually already been wound up, and is difficult and time-consuming to remove.
German Auslegeschrift No. 24 15 991 attempts to eliminate these defects using special adjusting means on the thread control device. These adjusting means are intended to prevent the thread breaking and being wound up on the take-up discs, although this cannot be ruled out.
In order to prevent broken thread being wound up, it is proposed in German Offenlegungsschrift No. 29 14 117 which corresponds to U.S. Pat. No. 4,325,314, to clamp the looper thread periodically in front of the discs, in order to be able to break it off should it start to be wound up. This additional device is, however, relatively expensive as it requires parts which move in synchronism with the stitch formation.
In a thread control device as described in German Patent Specification No. 35 10 332 which corresponds to U.S. Pat. No. 4,633,795, knives are disposed immediately adjacent to the discs beneath the cover plate in a holder, the cutting edges of which knives are substantially parallel to the rotational axis of the discs. These knives are used to cut off the looper thread, which is taken up by the discs when a thread break occurs, when said thread reaches the region of the knives, and hence to avoid winding up. These knives are prone to wear and are ineffective when their cutting edge is blunt. Furthermore, they must be very precisely adjusted in order to be effective.